Rough time with this JMT poly 80% AR on drill press

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  • TravisVerve

    Member
    May 3, 2021
    25
    So i got end mill bits and started the attempt to get the pockets milled out with my cheapie 8in Drill Press and it is not going well.
    I'm not attempting lateral milling b/c i have no x - y just a simple vise and i don't think the drill would survive it.
    Even going sloooowwww just doing downward plunge 1/8th" at a time the thing wants to jump around and get wobbly. Im using the 760 rpm config like JMT recommends.

    I am tapped out on $ or I would just spend the $800 for a small milling rig like you can get at Harbor Freight. But I only plan on doing 2 maybe 3 of these. I've looked at a ton of YT videos and not finding any jems of wisdom.

    Any tips?
     

    E.Shell

    Ultimate Member
    Feb 5, 2007
    10,242
    Mid-Merlind
    Please be VERY careful trying to apply side-pressure milling with a drill press. Plunge-cut as much as possible and keep any side cutting pressure very low. Many/most real drill press chucks are taper-fit and can be dislodged while rotating by applying side pressure. If the chuck pops free while running, you can be hurt and/or break stuff.

    Everything needs to be SOLIDLY clamped to prevent chatter. Make sure all of the hardware is tight on your drill press-head/table/base.

    Feed rate is more about speed of advance than it is about depth/amount of cut in a step. A fast plunge to 1/8" is a lot to bite, while a slow advance at the right speed can be steady to any depth, until the cutter flutes fail to clear chips. Maybe try slowing down your approach and give the tool time to cut.

    While I understand you are faithfully following directions, 760 RPM seems a little low to me for a relatively soft material like polymer. You might try a slightly higher speed and see if it makes your cuts smoother.

    IANAM (I am not a machinist. (But I have destroyed my share of tools and parts))
     

    Joseph

    Ultimate Member
    MDS Supporter
    Oct 13, 2009
    2,758
    Clinton MD
    Two things may be at issue here.

    1 make sure your end mills are center cutting. Some are not and you will have a b!tch of a time plunge milling with them.

    2 your drill press may have too much radial slop in the bearings allowing the end mill to dance around. Even worse if #1 is also at play.
     

    brianns

    Ultimate Member
    MDS Supporter
    Dec 29, 2015
    3,559
    Montgomery County
    I recommend not trying side pressure with a drill press. It could jump and bugger up your project, not to mention other unsafe outcomes. Plunge then move replunge then move, etc. It's tedious but the better way to do it.

    You would need something to hold the press at depth and a good vise block to hold your piece and feed it against the bit. But that is what you need. If you only have drill press, don't try hand feeding. I guess that's what I'm trying to say.
     

    TravisVerve

    Member
    May 3, 2021
    25
    Two things may be at issue here.

    1 make sure your end mills are center cutting. Some are not and you will have a b!tch of a time plunge milling with them.

    2 your drill press may have too much radial slop in the bearings allowing the end mill to dance around. Even worse if #1 is also at play.

    This is a bingo. They are not center cutting. (i thought they were and too dumb to verify)

    All the other advice is good I def will only do the plunge cuts b/c this press is el-cheapo but I will have to get the right bit before I can try anything else. Glad I stopped. I dont want to rush to failure..

    So I should go ahead an just use the regular 3/8 drill bit to get most of the material out first?
     

    Rambler

    Doing the best with the worst.
    Oct 22, 2011
    2,162
    This is a bingo. They are not center cutting. (i thought they were and too dumb to verify)

    All the other advice is good I def will only do the plunge cuts b/c this press is el-cheapo but I will have to get the right bit before I can try anything else. Glad I stopped. I dont want to rush to failure..

    So I should go ahead an just use the regular 3/8 drill bit to get most of the material out first?

    End mills that would be used for this are flat on the end as opposed to a drill which is pointed. an array of drilled holes leaves the bottom rough as a corn cob and can cause the finish cuts taken with an endmill to be rough as well.

    Overall, besides the correct cutting tools, rigidity is what you are striving for. The work (piece you are cutting), tool, everything should be prevented from moving or vibrating as much as possible.
     

    brianns

    Ultimate Member
    MDS Supporter
    Dec 29, 2015
    3,559
    Montgomery County
    This is a bingo. They are not center cutting. (i thought they were and too dumb to verify)

    All the other advice is good I def will only do the plunge cuts b/c this press is el-cheapo but I will have to get the right bit before I can try anything else. Glad I stopped. I dont want to rush to failure..

    So I should go ahead an just use the regular 3/8 drill bit to get most of the material out first?

    I vote to get the end mills and use them. I used the ones from my polymer 80 kits. BTW I milled a JMT 308 lower. I like their product but think it would be better to have been able to drill the selector/pin holes from both sides of the receiver. I think I may have drilled my hammer/trigger pins off in some way. I haven't figured it out yet.
     

    TravisVerve

    Member
    May 3, 2021
    25
    Thought I'd update this post. Took me quite a while but I found an XY vise for $45 so I got it and took hella time to mill out the lower. Assembled yesterday and checked for trigger reset and all that jazz. Seems to work. Mated with the upper no prob, no slop, no binding. (Bolt catch roll pin was a treat to put in) Dryfire and ejection with dummy good.
    Can't wait to pick up my mags from Carbon and Steel in York and give it a go.

    While I was toiling away I got a G17 Poly 80 and Patmos full kit and put a 17 together. That was a breeze. Test fired yesterday. Light strike on the very first round and FTF on about the 30th but otherwise really accurate. I was surprised because I am a terrible shot... lol.

    Now for lower number 2 now that I have a clue
     

    brianns

    Ultimate Member
    MDS Supporter
    Dec 29, 2015
    3,559
    Montgomery County
    While on the subject-

    I didn't mention this before. Polymer80 recommends(I first read it in their instructions) following a counter clockwise path as you go around the inside of the fire control pocket. I found that to reduce chatter on my press. I think its because it works better due to the direction of rotation of the drill bit. For example, go from back to front along the left side of the receiver then across behind the mag well. Then front to back on the right side, etc.

    For those of us with cheapo drill presses its probably more important to follow this advice. It might just be good practice to begin with.
     

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    5d Tactical jig and router kit. Far cheaper than a chinesium micro mill and much better results (indeed, 5d has results that rival my mill and is more “idiot proof”)
     

    Joseph

    Ultimate Member
    MDS Supporter
    Oct 13, 2009
    2,758
    Clinton MD
    While on the subject-

    I didn't mention this before. Polymer80 recommends(I first read it in their instructions) following a counter clockwise path as you go around the inside of the fire control pocket. I found that to reduce chatter on my press. I think its because it works better due to the direction of rotation of the drill bit. For example, go from back to front along the left side of the receiver then across behind the mag well. Then front to back on the right side, etc.

    For those of us with cheapo drill presses its probably more important to follow this advice. It might just be good practice to begin with.

    Look up climb milling VS conventional milling. That is what you are experiencing. When you are getting the chatter you are climb milling.
     

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    brianns

    Ultimate Member
    MDS Supporter
    Dec 29, 2015
    3,559
    Montgomery County
    Thanks for those diagrams. The counter clockwise pattern keeps you from climb milling.

    To second the above, I did one aluminum 80% lower on a 5D Tactical Jig recently and liked it a lot.
     

    Rambler

    Doing the best with the worst.
    Oct 22, 2011
    2,162
    A counter clockwise tool path will produce climb milling. What you described, back to front on the left side, is a clockwise path resulting in conventional milling.

    Sent from my SM-G960U using Tapatalk
     

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